1. Field of the Invention
This invention relates to a sandwich structure having an excellent impact resistance, and more particularly to a sandwich structure comprising two same or different outer plates and an intermediate layer interposed therebetween. To be more concrete, the invention relates to a sandwich structure having an excellent impact resistance which can prevent the penetration of solid over a wide temperature range even when the solid comes into collision with the outer surface of the sandwich structure at a higher speed or a higher acceleration speed and can lessen the scattering of broken pieces in all directions when the structure is broken by the collision of the solid.
2. Related Art Statement
In this type of the sandwich structure, various materials such as metals, ceramics, organic high polymers and so on can be used as an outer plate. In case of using ceramics, particularly transparent glass, when a transparent resin is interposed between the transparent glasses to form a sandwich glass, since the glass is conspicuous in the brittleness, the impact resistance of the sandwich glass is fairly poor. Therefore, it is demanded to provide optical transmission sandwich glasses having an improved impact resistance as a windshield member for buildings, airplanes and vehicles, particularly automobiles. Heretofore, the sandwich glasses have been manufactured by using a transparent resin having a large elongation at about room temperature, such as polyvinyl butyral resin or the like as an intermediate layer. In the actual production, however, it is required to enhance the impact resistance as far as possible in order to prevent the penetration of driver's head through the windshield glass for automobile in collision accident. For this purpose, it is usually attempted to elaborate various special plans.
For example, in order to improve the impact resistance, the sandwich glass is produced by intentionally decreasing the bonding force between outer glass plate and polyvinyl butyral resin layer, because the penetration resistance is poor when the bonding force is high. In this method, the polyvinyl butyral resin layer is left to stand in a room held at a constant humidity to incorporate a given amount of water into the layer and then contact bonded to the glass plate by heating, whereby the bonding force is restrained at a low value. According to the above method, the productivity is lowered, and also the falling-off and scattering of sharp broken glass pieces can not completely be prevented when the glass is broken by the collision, because the bonding force inherent to polyvinyl butyral resin layer is restricted to a low value in use as a defect of product quality. That is, it is impossible to simultaneously and completely satisfy the penetration resistance and the performance for preventing the scattering of glass pieces. As another great drawback, the penetration resistance improved by the control of water content is restricted within a very narrow temperature range. In general, the penetration resistance is maintained only at a temperature within a range of 15.degree. C..about.25.degree. C. and lowers at a temperature outside the above range. Particularly, the penetration resistance rapidly lowers at a temperature above 30.degree. C. Since the sandwich glass is actually used under conditions over a wide temperature range, the above drawback is a serious problem to be solved, which is overlooked without knowing why up to the present.